The present invention relates to a plastic screw cap with tamper-evident ring.
Caps of the above type are already commercially known. They comprise a cylindrical cup which is threaded internally so that it can be screwed onto the top of the container (bottle). A so-called tamper-evident ring is attached to the rim of the cup by means of breakable bridges and is internally provided with engagement elements constituted by flaps or by a collar which, when the cap is applied so as to close the container, engage below an annular retention ridge of the container. By unscrewing the cap, the flaps or the collar abut against the annular ridge and retain the ring, while the resulting axial traction force breaks the bridges.
However, in currently commercially available caps breaking bridges entails a significant physical effort, owing to the fact that the traction force during unscrewing must overcome the mechanical contrast opposed by all the bridges simultaneously. In practice, the unscrewing force must be such as to break a cross-section which is equal to the sum of the cross-sections of the bridges.